Electrical energy storage systems are of use both to producers and consumers of electrical energy. Storage systems for electrical energy may include storage batteries or other chemistry-based storage systems, capacitors or other electrically-based storage systems, thermal storage, or mechanical energy storage systems. Mechanical energy storage systems may include gravity-based storage systems, or inertial systems. Inertial systems may include flywheel systems.
A typical flywheel system consists of a flywheel in the form of a rotating mass that shares a common shaft with a rotor of a motor/generator unit. The rotating mass may include a material that is sufficiently dense and strong to effectively store the energy while remaining intact and operation. For example, the rotating mass may include steel, a composite material, or a combination of such materials. The motor/generator unit functions as a motor during a charge phase of the system and as a generator during a discharge phase.
During a charge phase, electrical power to be stored by the flywheel is provided to the motor/generator unit from a generating system or from an electrical grid in the form of an electrical current. The motor/generator unit then functions as an electrical motor. The current causes the rotor of the motor to generate a positive torque that provides angular acceleration to increase the rotational velocity, and thus the rotational kinetic energy, of the flywheel. The flywheel reaches a desired rotational velocity at which the flywheel is storing a desired quantity of energy. (Since rotational kinetic energy is proportional to the square of the angular velocity, flywheels are typically designed to spin at high speed.) The charge phase then ends.
After the charge phase, a store phase (typically longer than the charge phase) may begin. During the store phase, the motor/generator unit may be disconnected from any external electrical circuit, being thus placed in an idle mode. Thus, rotational inertia of the flywheel causes the flywheel to continue to rotate, storing the energy as rotational kinetic energy of the flywheel. During the store phase, various frictional forces may act to slow the speed of rotation of the flywheel and to cause loss of the stored energy. Therefore, during the store phase, current may be provided intermittently and for brief periods to the motor/generator unit to restore lost energy.
When power is to be extracted from the flywheel system, a discharge phase is entered. During a discharge phase, energy that is stored in the flywheel is converted to electrical power and made available for use (e.g., via an electrical power grid). The motor/generator unit is connected to an external electrical circuit and functions as a generator. Rotation of the flywheel turns the rotor of the generator and generates electrical power while applying a decelerating or braking force to the flywheel. The discharge phase may continue until there is no longer a need for the stored energy. The flywheel system may then revert to the store phase. In other cases, the rotational velocity of the flywheel may be reduced during the discharge phase to less than a minimum velocity (e.g., below which the system is no longer capable of generating usable electrical power). In this case the system may enter a wait phase.
During the wait phase, the flywheel may be stopped or may be rotating at a minimal velocity. The motor/generator is disconnected from external circuits and placed in an idle mode. The wait phase may continue until electrical energy is available to charge the flywheel again.
For an energy producer, energy storage enables provision of electrical power to the electrical grid at a constant rate. For example, the rate of generation of electricity using renewable sources such as solar, wind, or tidal power may vary as the power source varies. Thus, at times when electrical power production exceeds demand, excess produced energy may be stored. On the other hand, at times when demand for electrical power exceeds production, the stored energy may be provided to the electrical grid for use by consumers. Similarly, energy storage may enable electrical power production at a constant rate, regardless of momentary demand. Thus, electrical power may be generated without a need for (e.g., fuel based) generators that are operated only when demand is high (and may produce more carbon or pollutants than the generators that are operated constantly).
Similarly, a storage system may be used by a consumer to save energy costs. For example, the cost of electrical power from the grid may vary periodically. An electric power rate structure may charge more for electrical power during peak demand hours and less during off-peak hours (e.g., a rate during peak hours may be triple the rate during off-peak hours). A consumer with an energy storage system may thus buy electrical power during off-peak hours and use the saved energy during peak demand hours.
As compared with other energy storage techniques, systems, or methods, a flywheel provides some advantages. For example, the number of charge/discharge cycles is virtually unlimited, limited only by the wear of the mechanical parts. The amount and frequency of required maintenance may thus also be low as compared with other systems. Flywheel systems may also be relatively insensitive to environmental factors such as temperature changes. A flywheel system does not require use of hazardous materials, does not emit harmful gasses, and components of the system may be recyclable at the end of the useful life of the system.